1. Field of the Invention
The present invention pertains to thermally activated electrochemical cells, and more particularly relates to thermal battery cells and still more particularly to thermally activated battery cells including a novel electrolyte system that is free of chloride ions.
2. Background of the Prior Art
Thermal batteries are used in all types of military devices that require long shelf lives (up to 20 years), and rapid activation (less than one second), and high energy and power densities. Prime application examples are air-to-air missiles such as the Sidewinder missile. Modern technology is placing increasing heavy requirements for electrical power in many weapon applications, thus thermal batteries with higher cell voltages and higher energy and power densities are presently in demand. Because of its strictly military applications, thermal battery technology has evolved much slower than commercial battery technology. Thermal battery development began with the Germans in World War II where Ca/LiCl—KCl/CaCrO4 (2.8 V, 35 Wh/Kg) systems were used in the V-I and V-II rockets. The next major advancement was in 1980 with the introduction of the following thermal battery system: Li—Fe/LiCl—KCl/FeS2 (2.0 V, 55 Wh/Kg). Later variations included the use of Li—Si and Li—Al anodes and the all-lithium electrolyte of LiCl—LiBr—LiF. The Li—Fe anode consist of liquid lithium (mp=181° C.) immobilized by iron powder and is often referred to as the LAN anode (Liquid Anode).
Thermal batteries have mainly focused on the use of LiCl—KCl electrolytes and other halide variations ever since their inception over sixty years ago. The LiCl—KCl eutectic has a high melting point of 352° C., and actual thermal battery operations require temperatures greater than 450° C. These high operating temperatures necessitate large amounts of energetic pyrotechnics such as Fe+KClO4, and the use of insulating materials to maintain the operating temperature over the required lifetime of the activated thermal battery as well as for the protection of nearby electronics from the heat evolved by the thermal battery. Furthermore, the use of LiCl—KCl and related halide electrolytes prevents the use of modern high-voltage metal oxide cathode materials such as V2O5, LiCoO2, and LiMn2O4. The aggressive attack by chloride ions at high temperatures results in the rapid destructions of metal oxide based cathode materials.
Molten nitrate electrolytes offer many advantages over LiCl—KCl and other halide-based electrolytes. For example, the LiNO3—KNO3 eutectic melts at 124° C.—more than 200° C. lower than LiCl—KCl. Therefore less pyrotechnic material is needed to activate the molten nitrate thermal battery and less insulating material is needed to maintain the lower operating temperature of this thermal battery. There would also be much less heat radiation from the molten nitrate thermal battery into any nearby electronic components of the missile or other weapon devices.
The major advantage offered by molten nitrate electrolytes for thermal batteries is that they are compatible with high-voltage metal oxide based cathode materials such as V2O5, PbO2, LiCoO2, and LiMn2O4. In fact, the oxidizing nature of molten nitrates favors the formation of metal oxide materials. The use of metal oxide based cathode materials can yield cell voltages of over 3 volts when used with lithium cathodes, whereas the present thermal batteries such as Li/LiCl—KCl/FeS2 yield only 2.0 volts per cell. This increase in cell voltage alone to 3.0 volts offers more than a 50% increase in the energy density of the thermal battery. For example, a fifteen-cell stack that delivers 30 volts can be replaced by a ten-cell stack if each cell yields 3.0 volts instead of 2.0 volts. Thus, a significant decrease in the battery weight and space requirements can be realized. In addition, less insulation and less pyrotechnic materials are needed in the molten nitrate thermal battery. In addition, the much lower operating temperatures offer longer operating lifetimes for molten nitrate electrolyte-based thermal batteries.
The major problem that must be solved before molten nitrate based thermal batteries become practical is the minimization of unwanted gas-production reactions within the battery. The solution to this problem is the object of this invention. With this invention, a major increase in cell voltages, energy density, and power density is feasible for thermal batteries using molten nitrate electrolytes.